Mapping the Ocean-Continent Transition at Rifted Margins using Satellite Gravity
Inversion Incorporating a Lithosphere Thermal Correction
Neil W. Hurst and Nick J. Kusznir
Department of Earth and Ocean Sciences, University of Liverpool,
Liverpool, L69 BX, UK
(n.kusznir@liverpool.ac.uk)
Satellite gravity data and digital bathymetry provide powerful data sets for investigating the
transition from continental to oceanic crust at rifted continental margins. The inversion of
satellite gravity data at rifted continental margins to map crustal thickness variation and
predict the location of the ocean-continent transition (OCT) requires the incorporation of a
lithosphere thermal gravity anomaly correction for both oceanic and continental lithosphere.
Oceanic lithosphere and stretched continental margin lithosphere produce a large negative
residual thermal gravity anomaly (up to -380 mgal), for which a correction must be made in
order to determine Moho depth. The gravity inversion using the thermal gravity correction
predicts oceanic crustal thicknesses consistent with seismic observations, while that without
the thermal correction predicts much too great oceanic crustal thicknesses. The lithosphere
thermal model used to predict the gravity anomaly correction may be conditioned using
magnetic isochron data to provide the age of oceanic lithosphere. The resulting crustal
thickness determination is however sensitive to errors in the magnetic isochron data. A new
method of inverting satellite gravity at rifted continental margins to give crustal thickness,
incorporating a lithosphere thermal correction, has been developed which does not use a priori
information of the location of the OCT using magnetic isochron data and provides an
independent prediction of OCT location. Where oceanic isochrons are believed to be reliable
the lithosphere thermal gravity correction may be determined for oceanic lithosphere from
ocean lithosphere age, and for the thinned continental margin lithosphere using margin rift age
and beta stretching estimates iteratively derived from crustal basement thickness determined
from the gravity inversion. Where ocean isochrons are unknown or unreliable, all lithosphere
is assumed to be initially continental and a uniform lithosphere stretching age is used
corresponding to the time of continental breakup. The thinning factor produced by the gravity
inversion is used to predict the thickness of oceanic crust using an empirical relationship to
predict the thickness of oceanic crust from lithosphere thinning factor parameterised by a
critical thinning factor for the start of ocean crust production and maximum oceanic crustal
thickness. This new modified form of gravity inversion with embedded thermal correction
provides an improved estimate of rifted continental margin crustal thinning and an improved
(and isochron independent) prediction of OCT location. The new gravity inversion method
of OCT mapping has been successfully applied to several case histories including the
northern N. Atlantic. Predicted Moho depth and crustal thinning, using the gravity inversion
with embedded thermal correction, compare well with those produced by wide-angle
seismology. This work forms part of the NERC Margins iSIMM project.